BRC 3000 Chassis
System: 625 Line
Original List Price : £000.00 ( to be ascertained)
Valves: None, all transistor
This 25″ Baird 8724 is quite a rarity, it used the first generation Thorn 3000 single standard chassis. As far as I know there are only two other survivors known to the vintage communities, of course if you know different please leave a comment below.
This is what it should look like, however you will see the one I found was shall we say, a little neglected!
The one I located was just a cabinet, I bought it from a nice fella in London who found it on the street, dumped with the rubbish. He took pity on it and gave it a home and briefly toyed with the idea of turning it into a fish tank.
Thankfully fate intervened in the form of a pregnant wife and he was forced to sell it on, which leads us back to fate and it coming to me. I had hoped to find one of these Baird sets but as I say they are so rare it was highly unlikely to ever happen. So when a chance of this came up regardless of condition I snapped it up.
So the condition? Well its a mammoth project, it is just an empty cabinet. The chassis is missing along with the CRT having had its neck snapped off. The scan coils gone as well as convergence yoke, blue lateral, chassis mount cross members all the internal panels such as IF, decoder, video, frame, line and power etc. The back panel is missing and the crt mask is damaged. What remained from the original set was the tuner and aerial input, on off, colour brightness, volume controls. the speaker. Well that’s a positive I suppose.
The Baird As Found
Looks good you think? all there…. Well all is not as it would seem
It had been completely gutted, and the CRT necked. All I was left with was the tuner, controls and speaker.
First thing to do is locate a replacement CRT as without this the project is a non starter. As luck would have it, I was put in contact with an Ex Trade engineer who had an unused re-gunned Mazda A63 200X.
A call to another Ex engineer located a BRC 3500 chassis frame with harness and over the next few weeks all the panels to rebuild the set were sourced some 3000 some 3500.
I set about making the missing horizontal chassis support rail, this fixes the vertical supports rail. Once installed the rail and runners installed were added. Picture shows 3500 chassis frame sitting on the support rails.
All the missing boards have been sourced, they came from various sources and sets, no doubt some components will need replacing. I assembled them on the 3500 frame. Made a track repair to the video board a C227 had been replaced with a monster 2uF 350V tacked on the underside. This resulted in the track breaking away, I removed the large cap, replaced it with the correct 2.2uF 63V on the component side then repaired the broken track underneath with some bridging wire.
Replacement CRT fitted, wired up the Yoke, I noticed on the PSU the 30v Zener in the stabilising circuit is missing along with C609 a 400uF 63V electrolytic. I didn’ have any suitable zeners in stock so ordered up a 1N5353 5W. Also found on the Convergence board the A1 switches have been removed and are shorted out, these need replacing. Also need to sort out the missing tuner plugs, sort out the missing degauss shield plug, check the LOPT is wired for the right kV,and do some final checks, mainly wiring checks. The blue lateral assembly had damage to one end, a lug which had a hair wire and two other was broken. I’ve made repairs but I’m not sure how good it will be until we have something on screen.
PSU module has had the electrolytics replaced and the 30V zener. Power applied, the set trips, found the Chopper transistor had failed. Installed a new one, same scanario, Chopper is OK. Tried the PSU in another sets 3000 chassis, same scenario, trips. PSU back in the Baird, removed 6 pin plug from underneath, still trips. Found dry joint on the 300V chopper collector and a Burnt and low resistor, still tripping.
Next is to isolate the collector and try further component tests to isolate the cause of the tripping.
Tracked the tripping issue. Once W621 and SCR957 was removed the tripping ceased. The SCR was tested on a bench supply and at 1.5V it would latch, when tested at 30V it would intermittently latch so appears to be failing under load. A new SCR was fitted and as a precaution the 72V zener W617 BZX61-C72 was also replaced.
Powered the PSU up still with the board isolated from the LTB and the chopper collector drive removed, the tripping has stopped. Now to reconnect the chopper, test and the LTB test, hopefully the tripping will not reappear.
Reconnected the feed to the chopper VT604 removed F603 and connected a dummy load to L603. Powered on set did not trip however the dummy load lit up very bright, 158V across L603 when it should be 60v. Investigation continues
Tracked the issue down to faulty new components, namely the new chopper transistors. Installed an old chassis-pull R2010B and voltages return to normal.
Now the PSU seems stable I hooked up all the interconnects and F603, powered the set and I have first sound. No first light but its a step in the right direction. The 60v rail is low at 27.8v and no EHT.
R907 was very hot indication that excessive current is being drawn. Replaced the Line output transistor but this did not resolve the issue. Removed the EHT transformer and performed a ring test, this revealed a very poor ring indicating shorted turns.
As a matter of precaution the EHT and line output transformers were replaced along with the tripler. Ring trace on the new EHT TX reveals a good ring.
After replacing the TX’s It was found that I had on wire wrong on T504 to T503, once rectified I had full 60V rail and EHT. Video board and sound produced nothing, a squirt of cleaner in A1 restore this switch and first light.
Next Test was to prove the I.F. board and tuner, that seems to be ok
Lots to sort out, A1 controls are quite high, I need to find some purity rings, sort the convergence, perform EHT set up. All the boards except PSU and LTB need servicing many awful caps are present.
Results after Setting EHT, Grid bias, Porch Bias, Clamp pulese level, Grey scale adjustment. Below is the grey scale test card. Looking much better now. However I still need to do Pre-set Brightness and Beam Limiter.
One outstanding fault is I don’t have any colour whatsoever coming out of the chroma/decoder. Out of interest I installed a known working 3000 chroma board into the Baird, this then gave me colour. This proves the Baird chroma board has the final remaining fault to track down.
The investigation will start with disbaling the colour killer and hopefully the results on screen will lead me to the true fault. Below is the picture of the Baird showing a colour picture (using another sets chroma board) for the first time, ah it could be 1969 all over again.
Fault finding on the Baird chroma board revealed an OC diode in the blanking circuit, namely W316. It would appear on even closer inspection today, W317 had also gone O/C, so both blanking diodes had failed. These were replaced with NOS OA91 diodes. This resulted in colour being restored, I’ve not done any setting up so you’re seeing what I saw when it came on but that’s the Baird now running with all its own panels.
A few more faults came to light after a soak test with the beam limiter, frame board and tuner. The tuners mechanical plastic stop had perished and were replaced as per the guide here. The other faults were addressed along with a convergence.
Finally a solution to the missing back cover. Making one would have been difficult but not impossible, however I decided to wait until something to turn up from a scrap set. The time came when a Dual Standard Korting cabinet appeared on E-bay. I contacted the owner and suggested that if the cabinet did not sell I would make a generous offer for just the rear cover. The cabinet did not sell and subsequently I obtained the cover.
This now forms the greater part of the back, all I need to do is adapt a bottom straight cover and small left side cover.
The Baird now running in the display room.
Receiving 2 UHF channels from my home analogue distribution system BBC1 showing Test card F with test card music via the IMOGen generator and on ITV via a DVD, Crossroads.